This invention is in the field of Stirling cycle engines and refrigerators, particularly the type where a reciprocating piston in a cylinder has a central bore through which extends a reciprocating displacer rod. Traditionally in these machines rubber rollsock or rolling diaphragm seals are used between the displacer rod and the bore of the piston, and between the bore of the cylinder and the outer surface of the piston. Since the piston and displacer reciprocate at the same rate but in an out-of-phase relationship, the axial distance between them constantly varies, and seals between the piston and displacer rod and cylinder respectively must be capable of cyclic extension and compression while remaining securely attached to corresponding surfaces.
The typical rollsock seal is an elongated rubber sleeve with one end rolled back toward the other end so that the overlying portions define a generally U-shape in a cross-sectional view, which is the area that cyclicly bends and reverse bends. To prevent the rollsock from stretching excessively during normal operation of the apparatus a quantity of substantially incompressible liquid such as oil lubricant from below the piston is maintained on the lower side of the rollsock opposite the side exposed to high pressure working gas. As long as the working gas pressure is greater than the liquid pressure, the rollsock will be urged toward the incompressible liquid. The liquid below the sock supports and prevents the sock from stretching excessively, and the pressure toward the liquid prevents the sock from forming folds or pleats in the direction away from the liquid.
With these rubber or equivalent composition rollsock seals a persistant problem is diffusion of the typical helium or hydrogen working gas through the rubber membrane into the supporting liquid. Such gas lost by diffusion must be removed from the liquid or the latter will cease to be incompressible. This gas diffusion problem is actually twofold, because the working gas lost by diffusion must be replaced by using the recovered gas or by other new gas. When the Stirling cycle device is an engine as contrasted to a refrigerator, the high operating temperatures contribute to deterioration of the rubber seals in addition to the diffusion problem.
Metal has been considered as an alternative to the rubber material of the rollsocks, because the metal is impervious to gas diffusion and the metal will not deteriorate at the elevated temperatures applicable hereto. Nevertheless, metal rollsocks are not feasible, because very thin metal, if configured as a rollsock, would rupture either from the pressure applied or from the reverse bending about small radii, and metal thick enough to avoid rupture would be too stiff. Practically any metal seal selected will introduce spring forces into the dynamic system which must be overcome or be considered in overall force and torque balancing and in overall efficiency.
Metal bellows have been used in various situation, primarily as flexible conduits which are thin and flexible where pressure is negligible and/or substantial movement is required, or are thick and stiff where pressure is great and/or movement is negligible or slow as in valves.
None of these known rollsock or bellows elements as known will solve the Stirling cycle engine and refrigerator problems discussed above. The present invention, however, involves metal bellows that may be very thin foil and that will operate successfully in Stirling engines or refrigerators with the advantage of rollsocks and the exlusion of the persistent problems.